492 THE GLACIAL LAKE AGASSIZ. 



cnistal temperntures would depress the laud while it was ice-covered aud 

 raise it wheu the ice was withdrawn depends on the ratios of contraction 

 and expansion of the underlying rocks. These ratios have been experi- 

 mentally determined in the case of various building stones, and computations 

 therefrom indicate that only a very small amount of subsidence and eleva- 

 tion of the land could be caused in this way. ^ The total elevation so 

 produced was probably not more than 50 feet in the southern part of the 

 Ked River Valley, and not more than 30 feet at Winnipeg; and its slight 

 differential effect would be in the opposite direction to that which has given 

 to the beaches of Lake Agassiz their northward ascent. This element in 

 the causation of the changes of elevation appears to be comparatively 

 insignificant in itself, and its small component in the oscillation of the shore- 

 lines would be opposed to that for which we are seeking an explanation. 



EPEIROGENIC MOVEMENTS APPARENTLY DEPENDENT ON GLAf'IATION. 



It seems to be very clearly indicated by the gradual diminution in the 

 northward ascent of the beaches, until the lowest and latest have nearly 

 the level of the present time, tliat these progressive changes of elevation 

 were directly dependent upon the departure of the ice-sheet, with which 

 great geologic event they were contemporaneous. As already noted in 

 Chapter V and on a foregoing page of this chapter, these changes were so 

 directly proportionate with the glacial recession that the northward ascents 

 of the successive beaches were at first referred to the diminishing gravi- 

 tation of the lake toward the ice-sheet; but, apai't from the inadequacy of 

 this cause, determined by Mr. Woodward's investigations, the great extent 

 of the highest beach and its relation to terminal moraines marking stages in 

 .the glacial recessif)n sufficiently demonstrate that other causes contributed 

 even more than ice attraction to produce the changes observed in the levels 

 of the beaches. 



There remain to be considered, as probable causes, first, the relation- 

 ship between the earth's crust and its interior which may have permitted a 



' T. C. CUamberlin in Sixth Annual Report, U. S. Geol. Survey, p. 302, and in paper read before 

 the Philosophical Society, Washington, March 13, 1886; G. K. Gilbert in Am. .lour. Sci. (3), Vol. 

 XXXI, p. 297, April, 1886, and in V. S. Geol. Survey, Monograph I, "Lake Bonneville," pp. 377,378. 



